1. Field of the Invention
This invention relates to an improved printed circuit board (PCB) and more particularly to a multi-chip module (MCM) interconnect structure or device which is fully programmable or customizable on the surface layer to meet application-specific needs.
2. Background of Relevant Art
PCBs or printed wiring boards (PWBs) are rigid or flexible single, double or multi-layered board having printed conductors placed upon or within the board material. A PCB is designed to receive separately manufactured electrical components and to interconnect those components into an overall circuit structure. The components or parts comprise integrated or discrete circuits well-known in the semiconductor arts.
Many PCBs are manufactured by the subtractive process in which conductive paths or lines are lithography formed upon a substrate material. Layers of lithography-formed lines are fabricated in successive steps necessary to achieve a multi-layered PCB. Each layer of conductive lines is laid out according to uniquely derived artwork designed to connect electronic devices placed upon the PCB. Artwork layout requires a significant amount of forethought and effort in order to ensure that, when reduced to a custom PCB, the PCB will perform according to its netlist requirements. If the netlist is determined to be improper or if slight modifications are necessary after the artwork and custom layers are formed, an entirely new netlist, including artwork and custom layers must be derived. Reduction of a netlist to a custom PCB is not only time consuming but also quite expensive. For reasons stated above, many manufacturers are focusing on multi-chip modules (MCMs) which are not fully customized at the fabrication stage. Instead, MCMs are mask produced according to a generic structure for most, if not all, of its layers.
Generally speaking, there are two types of MCMs: (i) semi-programmable MCMs or (ii) fully programmable MCMs. Semi-programmable MCMs generally utilize a pre-fabricated layer or layers arranged in an insulative substrate. The existing layers are generic to any component placement, configuration and netlist. To achieve design-specific interconnection, one or possibly two customized layers are fabricated upon the existing, generic layers. The custom layers are constructed according to the specific component arrangement and netlist, and are generally formed by the same lithography steps used to form a customized PCB layer. Since semi-programmable MCMs require customization of only the last one or two layers, manufacturing throughput is greatly enhanced relative to fully customized PCBs. While semi-programmable MCMs can implement a specific design in less fabrication time than a fully customized PCB, semi-programmable MCMs require more time to clean-room fabricate than a fully programmable MCM.
Fully programmable MCMs are ones which are fabricated according to a generic layout structure and are customized only upon the upper surface layer, generally at the site away from the location of the manufacturer. Fully programmable MCMs can therefore be customized without requiring lithography or clean room facilities. There are numerous types of fully programmable MCMs, some of which are described in U.S. Pat. No. 5,132,878 to Carey; U.S. Pat. No. 5,220,490 to Weigler, et al.; and, U.S. Pat. No. 4,888,665 to Smith.
Important goals necessary to achieve a fully programmable or fully customizable MCM are: routeability, testability and power distribution/availability. Routeability is often a measure of the efficiency by which a conductive line extends between target locations. Generally, the shorter the line the better. Optimal routeability implies that the surface target locations be configured as close to electrical components as possible. If the target locations are substantially displaced from the electrical components, long leads are necessary to connect the component to the target area. Thus, to optimize routeability, bonding pad locations must be arranged in a manner which can take into account any possible number, size, or shape of overlying electrical components.
In addition to routeability, fully customizable MCMs require ease of testability. It would be highly desirable to test each of the generically placed conductive lines prior to component placement on the interconnect structure and customization of that structure. As such, the generically placed conductive lines should be configured in a way that would allow the operator to test the resistivity of all of the lines by applying a test probe on only a few select locations. Provided the upper surface is properly programmed, pre-testing of the conductive lines ensures that the interconnect substrate will perform.
In addition to routeability and testability, fully programmable MCMs must also have readily available an evenly distributed set of potential conductors. Design-specific circuitry often requires more than one ground and one power potential. In many memory applications, two power potentials and two ground potentials are needed. A fully programmable MCM must therefore embody more than two potential conductors, and each potential conductor must exhibit a minimum resistance gradient from one end of the interconnect structure to the other. Accordingly, the potential conductors must be well distributed throughout the interconnect structure upon wide conductive paths. More importantly, each potential conductor must be arranged as close as possible to the bonding pad for delivering the appropriate potential to the sinking electrical component (or load). Thus, it would be desirable to have four possible potential conductors, and each potential conductor should optimally be available directly beneath a bonding pad arranged on the upper surface of the interconnect structure. Thus, the potential conductor can make direct connection to the bonding pad and carry the desired potential from the bonding pad to the electrical component with minimal resistive loss.